1. Field of the Invention
This invention concerns a system of arrays or antennas with high resolving power.
More specifically, the invention concerns detection of targets by a wave detection system, such as a sonar system, in which waves are transmitted and received after being reflected on a target or targets, by means of nonidentical transmitting and receiving arrays.
The main requirement in such systems is to obtain the highest possible angular resolving power, namely the ability of the array system to distinguish between two targets very close together given the size of the transmitting and receiving arrays.
2. Description of the Prior Art
Attempts have been made to obtain high resolving power in array systems in the prior art.
For example, more or less omnidirectional emitting arrays has been combined with a receiving array formed of transducers placed side by side along a line. This is known as a "full" receiving array.
Another method is to combine a full transmitting array with a full receiving array, in which case the reversibility of transducers makes it possible for a single array to act in turn as transmitting and receiving array.
A system comprising full transmitting and receiving array has a known resolving power provided the transducer amplitude and phase are constant. The same is true if the amplitude is constant and if the phase increases by a constant amount from one transducer to the next.
It is known to obtain a high resolving power, by using a transmitting array with uniformly distributed transducers, and a receiving array with only two end transducers. Both arrays are approximately the same length. By adding the signals delivered by these end transducers, it becomes possible to obtain a cosinusoidal radiation diagram, which acts as a multiplying factor for the radiation diagram of the full transmitting array.
It is also known to preform an angular beam for a full array, by applying signals detected by transducers to multiple-terminal phase-shifters. The various channels are obtained by adding dephased signals together.
It is known to use a system comprising transmitting and receiving arrays of the same length, the receiving array being full and the transmitting array being formed of two transducers at its ends. Signals detected by the receiving array are delivered to beam-forming circuits. In this device, transmitting is performed at two frequencies. The two transmitting transducers are in phase for the first frequency, while for the second they are in opposite phases. Half the preformed beams correspond to one of the frequencies, and the other half to the other frequency.
The angular width at 3 dB of the main lobe of the transmission-reception diagram is half that obtained with the same receiving array when there is only one transmitting transducer.
The drawback of this system is that secondary transmission-reception diagram lobes are high, particularly the first, which reaches a maximum level at 13 dB. Such a level of secondary lobes seriously affects the signal/noise ratio, particularly for underwater acoustics.
This invention overcomes such difficulties by reducing the distance between the two transmitting transucers and weighting received signals, thereby providing high angular resolution and low secondary lobe levels.